The present invention generally relates to intermodulation (IM) reduction in communications receivers and in particular in communications receivers comprising a front end stage for receiving a transmitted radio frequency (RF) input signal and a mixer stage for mixing the RF input signal and an oscillator signal so as to provide an intermediate frequency signal or baseband signal.
In a communications receiver a plurality of applied input signals with slightly different frequencies often create intermodulation signals due to non-linear characteristics of transfer elements in the communications receiver. The most important intermodulation signals are the second order and third order intermodulation signals, respectively. These intermodulation signals affect the receiver performance detrimentally when they fall into the traffic band. Since intermodulation signals become particularly strong in receivers when the signals of the interfering transmitters are very strong, prior art communications receivers are equipped with a real resistive pad. The impedance of such pads is equal to the path impedance of the RF signal. The pad attenuation is fixed and in some cases it may be manually switched in or out. The disadvantage of such fixed real resistive pads is: a) the intermodulation improvement versus receiver sensitivity degradation is not optimized; b) only at relatively low input signals (wanted and unwanted signals) the pad is applicable; c) the intermodulation protection is not satisfactory or not sufficient at high receiver input signals.
The present invention seeks to provide means for reducing or even suppressing intermodulation signals in the traffic band and to mitigate or avoid the above disadvantages and limitations of the prior art.